This invention relates to hot-rolling of steel and more particularly to a method for hot-rolling carbon and alloy steels in a continuous hot-rolling mill wherein water or other coolant is sprayed onto the surface of the steels between passes during hot-rolling to control the temperature of the steels during rolling and to produce steel products which have uniform metallurgical characteristics. The scale which forms on the surface of the steels during air-cooling to ambient temperature is uniform, smooth, fine-textured and relatively thin.
The conventional method for producing steel products, such as straight or coiled bars, coiled rods, wire and the like, involves hot-rolling carbon and alloy steels starting with at least billet sized metal blanks heated to an elevated temperature within a range of about 1950.degree. F. - 2150.degree. F. and continuously rolling the blanks in a continuous hot-rolling mill, such as a billet mill, bar mill, rod mill and the like. Usually the hot-rolled products off-the-mill have a surface temperature within the range of about 1850.degree. F. to about 2100.degree. F. Steel products hot-rolled by the conventional method on a continuous hot-rolling mill do not have uniform metallurgical characteristics and a non-uniform, thick, coarse and sometimes blistery scale forms on the surface of the steels during air-cooling to ambient temperature. The scale is difficult to remove by pickling in acid pickling solutions, for example, aqueous solutions of hydrochloric acid and the like, and the steels can be "burned" by the pickling solutions in areas where the blistery scale occurs.
It has, therefore, been a continuing aim of industry to produce steel products which have more uniform metallurgical characteristics than can be attained with conventional rolling practice and which have as little scale as possible formed on the surface of the air-cooled finished product. To achieve this aim, several methods of treating the steels after hot-rolling have been proposed and tried. One method that has been suggested and tried to avoid the development of objectionable scale has been to cool the as-rolled steel in an inert atmosphere immediately off-the-mill. This method was complex and was not particularly successful. Methods for treating the steel after hot-rolling are exemplified in U.S. Pat. No. 2,673,820 issued Mar. 30, 1954 to M. Morgan entitled "Treatment of Hot Metal Rods" and U.S. Pat. No. 2,516,248 issued July 25, 1950 to J. E. O'Brien entitled "Method and Apparatus for Cooling Rods" which are concerned with continuously hot-rolling steel billets to coil form and treating the coiled material to control the microstructure of the material and the scale formed on the surface during cooling. Morgan is directed to providing an air blast to cool the steel as it is being coiled on a reel, while O'Brien is directed to cooling the steel coil after removal from the reel. In either case, the cooling of the steel product is too late to effectively overcome the above mentioned problems, that is, control either the microstructure or the formation of scale on the surface. Other prior methods include U.S. Pat. No. 2,756,169 issued July 24, 1956 to J. H. Corson et al entitled "Method for Heat Treating Hot Rolled Steel Rods" which is directed to cooling hot-rolled steel rods rapidly to a temperature range of 900.degree. F. - 1300.degree. F. after being rolled to finish size. The steel rods are held within the above mentioned temperature range for a time to allow carbon to come out of the solution. After cooling, the rods are coiled. The cooling method includes sequentially quenching the rods in water and air after the rods come off the last roll stand of the finishing train to obtain the desired temperature. U.S. Pat. No. 3,001,928 issued Dec. 5, 1961 to J. B. Kopec et al entitled "Method for Heat Treating Hot Rolled Steel Rods" is directed to quenching steel rods in a water cooling chamber after the rods have been finish rolled and come off the last roll stand of the finishing train. The rods are coiled on a reel and are subjected to a second cooling step during coiling. U.S. Pat. No. 3,645,805 issued Feb. 29, 1972 to Hoffman et al is directed to depositing as-rolled steel rods or wire in overlapping turns or waps on a conveyor, maintaining the temperature of the steel to obtain a uniform grain size of not more than 5 and thereafter controlling the cooling of the steel to produce a microstructure of ferrite and pearlite. U.S. Pat. No. 3,389,021 issued June 18, 1968 to C. G. Easter et al and entitled "Process for Preparing Steel for Cold Working" is directed to water quenching the steel rods as they come off the last roll stand of a finishing mill. The steel rods are cooled to a temperature of 1450.degree. F. and the coils are laid in an overlapping configuration on a conveyor and are air quenched to 700.degree. F. The rods are then coiled. U.S. Pat. No. 2,747,587 issued May 29, 1956 to A. W. Strachan entitled "Apparatus for Quenching and Reeling Rods" and U.S. Pat. No. 2,880,739 issued Apr. 7, 1959 to J. A. Popp entitled "Apparatus for Quenching and Reeling Rods" are directed to treating steel rods after finish rolling. The steel rods are passed through a series of delivery tubes in which the rods are quenched in water after finish rolling in the last roll stand of a continuous hot-rolling mill, but prior to coiling on a reel. U.S. Pat. No. 3,604,691 issued Sept. 14, 1971 to William George Sherwood entitled "Apparatus and Method for Coiling and Quenching Rod" is directed to coiling steel rod as it comes off-the-mill on a reel and water quenching the coiled rod continuously while the rod is being coiled. The water quenching is accomplished by lowering the reel and the rod coiled thereon into a tank containing water. U.S. Pat. No. 3,735,966 issued May 29, 1973 to Bernd Hoffman and entitled "Method for Heat Treating Steel Wire Rod" is directed to alternately quenching and air cooling steel rod off-the-mill and prior to coiling. The alternate quenching and cooling prevents the formation of martensite in the steel.
While the above prior art practices have achieved some measure of success, the uniformity of metallurgical properties has not been fully achieved and the problem of scale formation has not been satisfactorily solved. The prior equipment and temperature control systems required to perform the treatment steps are frequently delicate, complex and expensive and generally cannot be installed on existing mills because of space problems. Attempts to control the temperature of the hot-rolled steel prior to air-cooling have included initially heating the steel to lower than normal rolling temperatures and also decreasing the rate of hot-rolling. While some beneficial effects have resulted, it has not been found practical or economically feasible to hot-roll steels in these prior suggested manners. Prior art attempts to control the finishing temperature of the steels in methods in which the steels are initially heated to relatively low temperatures of about 1500.degree. F. to 1800.degree. F. for hot-rolling have met with little success because the electric motors which drive the roll stands in the roughing and intermediate train became dangerously overloaded due to the strain of rolling "cold" steels. Overloading can usually be avoided by operating the line at low speeds but production is then lost and such an expedient is therefore not economically feasible.
During hot-rolling, the steel achieves high speed, for example, finishing speeds as high as 4,000 feet per minute in a bar mill and 10,000 feet per minute in a rod mill. The high speed at which the steel is hot-rolled is one of the reasons it has been deemed impractical, if not impossible, to treat the steel during hot-rolling. Therefore, prior art methods of achieving the above goals have been generally directed to treating the as-rolled steels off-the-mill.
Duplex grain structure in the as-rolled coiled steel occurs near the surface of the steel, because the steel retains heat, for a sufficient length of time to allow some grains to become enlarged at the expense of other grains. The overlapping loops of steel come in contactt with each other and areas in which heat is retained for long periods of time are formed, thereby causing grain growth and duplex grain formation in these areas.
The overall grain structure of the rolled steels also tends to be excessively coarse due to high finishing temperatures. Excessively coarse grain, like duplex grain, is difficult to spherodize anneal or cold form. The distribution of the spheroids formed during annealing also tends to be non-uniform. Acicular bainite forms in alloy steels during air-cooling after hot rolling at normal temperatures. Acicular bainite makes the alloy steels hard and brittle and difficult to cold work.
Heavy scale also forms on the as-rolled steel during air-cooling to ambient temperature. The formation of heavy, uneven, rough texture scale to due to the time required for the steels to cool from high finishing temperatures to ambient temperature. Coiled steel retain heat longer than steels which are exposed on all surfaces to a cooling medium. Hence in coil form, scale formation is accentuated. Of course, high finishing temperatures of straight bars also result in the formation of a heavy scale on the steel.
The above cited prior art while partially successful has not completely solved the problems of coarse grain structure, duplex grain formation, acicular bainitic formation and scale formation on as-rolled bars and rods.
It has been generally recognized that coarse grain structure, duplex grain structure and scale formation are due to the elevated finishing temperatures of the rolled steel. As explained above, prior art attempts to alleviate the problems of the prior art have been directed to cooling the steel off-the-mill or alternatively to the use of lower than normal initial hot-rolling temperatures in order to finish roll the steel at lower than normal finishing temperatures. Treatment of the steel after finish rolling has not been completely successful. Heating to lower-than-normal rolling temperatures is also impractical since the mill motors either overload, resulting in burned-out motors, or the steel must be rolled at a very slow speed, which is impractical.
We have discovered that the elevated finishing temperatures off-the-mill are caused by excessive heat generated in the steel being hot rolled during its passage through the intermediate and finishing train. Coarse grain structure is thus initiated before the metal leaves the mill.
We have discovered, furthermore, that it is possible to roll steel at normal roughing temperatures at the same or increased rates of speed to produce a product off-the-mill which is free of duplex and coarse grain structure and which has a thin, uniform scale formed thereon.